The National Aeronautic and Space Administration (NASA) has determined a need to develop a long life plasma generator (source) for use as an ion propulsion device (e.g., an ion thruster) in long duration space missions. Requirements for the plasma source include: long life without maintenance, low power operation, high plasma density, and high plasma uniformity across its diameter.
Propulsion systems capable of providing very high specific impulse (6000-9000 seconds) are desirable as primary propulsion options for far term missions to the outer planets and beyond. Gridded ion thrusters systems are capable of satisfying such mission requirements. Ion thruster systems used for such an application will be required to operate continuously for perhaps as long as 5-10 years. Such long continuous operation times place stringent lifetime requirements on thruster components and subsystems. In general, thruster lifetime is limited by essentially four potential failure modes: 1) discharge cathode failure, 2) neutralizer failure, 3) ion optics failure and 4) electron backstreaming. Failure modes 1 and 2 are related primarily to hollow cathode failure. Failure mode 3 is related to screen and accelerator grid degradation via erosion as well as grid to grid shorts arising from such erosion phenomena. Electron backstreaming becomes more and more of a problem as accelerator grid apertures widen due to erosion. High levels of backstreaming electrons can destroy the discharge cathode. One potential solution to this problem is the use of a magnetic grid. Potential design solutions also exist for increasing the lifetime of the ion optics such as the use of different electrode materials such as titanium or carbon or by simply increasing the electrode thickness. Hollow cathode failure can occur after prolonged operation due to physical erosion or to the depletion of barium in the insert brought on by barium diffusion and subsequent evaporation or the formation of tungstates that tie up the barium. Because of such phenomena, ion thruster hollow cathodes have a life of order 28,000 hours, which is not sufficient for those missions requiring more than three years of continuous thruster operation. The present invention focuses on the elimination of the discharge cathode and its failure modes.
NASA Glenn Research Center (GRC) has initiated an effort to eliminate the lifetime issues associated with conventional hollow cathodes by investigating electrodeless plasma production approaches. Microwave electron cyclotron resonance (ECR) discharge was selected as the plasma generation method based on a parametric comparison with other electrodeless plasma production approaches. In the ECR process, electrons resonantly absorb energy from an imposed electromagnetic field. The input radiation field is set at the electron cyclotron frequency. At this resonance, the electrons can gain energy continuously. This resonant process takes place on surfaces of constant magnetic field B that are established by the magnetic circuit. B=875 G for a microwave frequency of 2.45 GHz (Giga-Hertz). The hot electrons produced during this process ionize neutral gas, thereby generating the discharge plasma completely electrodeless. This approach has been successfully implemented on the Institute for Space and Astronautical Sciences (ISAS) MUSES-C asteroid rendezvous spacecraft in which a 400 W (watt) microwave ion thruster will be used for primary propulsion. Compared to the present invention, the MUSES-C thruster device is considerably smaller (10 cm beam diameter); it operates at 4.2 GHz (a more expensive frequency range, since higher frequency microwave tubes are less efficient than 2.45 GHz tubes); and primarily small volume, ring ECR plasmas are produced in the device. Also, the MUSES-C device suffers from cut-off issues associated with the way the microwaves are launched into its chamber (its microwave input port can be obstructed by the formation of a “throat” plasma in the port); the plasma source size is inherently small owing primarily to its higher frequency; power required to create a plasma ion is higher because ECR takes place closer to magnet rings, thereby increasing plasma losses to walls; and the device is inherently power limited because of the aforementioned throat plasma problem. The present invention is intended to improve upon all these limitations of the prior art such as the MUSES-C thruster.
Electromagnets can be used to produce the magnetic fields needed for ECR plasma generation, but power demands of electromagnets preclude their use for low power space propulsion. Furthermore, electromagnet coils produce magnetic fields that are highest at the coil axis. Magnetic field geometries more suitable for the formation of uniform ECR plasmas and improved plasma containment can be obtained with permanent magnet arrangements. Efficient plasma containment is possible with permanent magnet arrangements.
The application of permanent magnet ECR to generate an ion thruster discharge plasma has been investigated by NASA in the past. A cylindrical 30 cm (centimeter) diameter, permanent magnet ECR ion engine with power up to 700 W (watts) was studied by a NASA GRC contracted activity in the 1980s as a study looking into the possibility of using ECR for ion thrusters. The conclusion of the study was that rather than the permanent magnet approach tested, an all electromagnetic configuration might perform better, given that the permanent magnet version never could be optimized. During this effort, ECR plasma sources with discharge losses less than 150 W/A (watts per amp) at 5 GHz were tested. However, this device did not generate uniform plasmas (a consequence of its microwave injection scheme), and there was difficulty reaching predicted plasma densities. Additionally, it operated at 5 GHz, an expensive frequency range; the method of microwave injection made it inherently difficult to generate uniform, larger area plasmas; and the device tended to “mode jump” (i.e., the plasma was not stable at all conditions). The present invention overcomes these difficulties and achieves a well optimized permanent magnet configuration.
The present invention disclosure is based on more recent microwave ion thruster research activity at NASA GRC that focused on the development of a higher power ion thruster system (40 cm diameter, 5 kW (kilowatt)). The goal of this activity was to develop an extremely long life version of the NASA Evolutionary Xenon Ion Thruster (NEXT). The early stages of this development activity have utilized 2.45 GHz as the operating frequency. This operating frequency affords one a cost effective means of mapping out general operation characteristics of a multipole plasma source. Issues such as magnetic circuit optimization, DC isolation, impedance matching, and power injection issues can all be studied using this test-bed plasma source. The beam current design goal for this 2.45 GHz ECR test-bed ion thruster was 1.1 A at a microwave input power of 300 W or less.
The maximum extractable current from a plasma is proportional to the product of the plasma density and the square root of the electron temperature. Thermal background electron temperatures are expected to range between 2 and 5 eV. To first order, the maximum plasma density obtainable depends primarily on microwave excitation frequency. The plasma in the ECR zones can reach a maximum density determined by the condition where the microwave frequency equals the local plasma frequency. Normally, at plasma frequencies higher than the microwave frequency, the microwaves are no longer absorbed, but reflected instead. Scaled up embodiments of the present invention will require a microwave frequency of 5 GHz or greater to achieve the required beam currents for the 5 kW operating condition. The higher frequency will also significantly reduce the discharge losses. Teachings from the present 2.45 GHz source embodiment will be implemented in the 5 GHz design.
The prior art evidenced in patent literature shows various microwave, permanent magnet, ECR plasma sources, but they suffer from limitations that the present invention overcomes.
US Patent Application Publication 2003/0006708 (Leung et al.; 2003) discloses a compact microwave ion source with permanent magnet rings (12). Gas in the 10 cm diameter chamber (14) is ionized by electron cyclotron resonance to produce plasma. A microwave antenna (22) is immersed in the plasma. The plasma density can be increased by boosting the microwave ion source by the addition of an RF antenna (42).
U.S. Pat. No. 5,975,014 (Dandl; 1999) discloses a large diameter, permanent magnet ECR plasma source with high density and uniformity. It uses a coaxial resonant multi-port microwave applicator (12) to introduce microwave power uniformity. An essential design element of the coaxial resonant multiport antenna array (60, in the plasma chamber—see FIGS. 2-3) is the geometric configuration of individual radiating stubs (62), that are formed in openings (63) in an outer conductor (64) of a coaxial line or coaxial waveguide (66) coupled to the power source (54), together with the spacing of successive stubs (62).
U.S. Pat. No. 5,133,825 (Hakamata et al.; 1992) discloses a plasma generating apparatus wherein a pair of hollow diskshaped rare earth magnets (21a, 21b) are provided around an outer periphery of a vessel wall (20) of a plasma generating chamber (1). A microwave transmissive window (24) is provided in one end plate of the plasma generating chamber, while a waveguide (3) is installed adjacently to the window, and a microwave generator (10), such as a magnetron, is provided on the waveguide. Hakamata's FIG. 5 illustrates an apparatus provided with a plurality of microwave generators (10) and preliminary plasma chambers (91) with respect to one main plasma chamber (94), wherein the size of the main plasma chamber is enlarged in diameter by increasing the number of the preliminary plasma chambers (91). Hakamata's FIGS. 6, 10, 12, and 13 illustrate several arrangements of apparatus wherein the cross-sectional area of the plasma generating chamber is substantially continuously increased from the waveguide-connecting side to the ion extraction side. Accordingly, the average intensity of the magnetic fields inside the plasma generating chamber becomes maximal at an axial position offset to the waveguide, and the magnetic field intensity decreases gradually toward the ion-extraction side, thereby expanding the plasma diameter. In addition, the plasma which is generated in the plasma chamber is pushed to the ion-extraction side by virtue of the action of the gradient in the magnetic field intensity.
U.S. Pat. No. 5,620,522 (Ichimura et al.; 1997) discloses a microwave plasma generator wherein a permanent magnet ring (3) forms an intense magnetic field exceeding an intensity of an ECR magnetic field at a microwave exit (6a) of a dielectric body (6) in an electron heating space chamber (1). The intensity of the magnetic field formed by the permanent magnet (6) decreases rapidly to zero at a point (14) in the vicinity of the boundary of the electron heating space chamber (1) and a plasma generating space chamber (2). A gas introducing means (7) is shown at the top of the plasma generating space chamber (2).
It is an object of the present invention to overcome limitations of the prior art. For example, microwave antennas, electrodes, and such should be eliminated from the plasma chamber where they can erode, thereby producing contamination and reducing lifetime of the device. For example, dielectric bodies (microwave windows) between the microwave source and the plasma chamber should be eliminated because in applications with metal vapor in the plasma, deposition of films on the dielectric window would prevent microwaves from entering the discharge chamber and/or from coupling into the source. (It should be noted that conducting vapor fluxes are intentionally part of various plasma processing environments, and can also occur during ion thruster operation due to low level grid sputtering. Furthermore, a microwave window can itself be a contamination source.)